Digital communication systems are steadily being replaced by optical systems to move information around more efficiently and with significantly higher bandwidths. As bit rates increase, the communication distances where optical solutions outperform their electronic counterparts is quickly approaching distances of up to a few meters. The use of optical integration has accelerated this movement to photonics both in the Indium Phosphide and Silicon integration platforms.
Separately, public sector systems and private sector systems are quickly expanding in bandwidth as the use of the electromagnetic spectrum expands from the traditional frequency bands below 18 GHz to frequencies up to and exceeding 110 GHz. The rapid advancement of electronic integrated circuits has enabled even an unsophisticated competitor the ability to design highly effective systems that can operate outside of these traditional frequency bands. Analog photonics in the form of fiber optics and discrete components have shown much promise in dealing with this bandwidth problem. However, the analog performance of even fiber optic solutions for these problems are just now beginning to have the necessary radio frequency (“RF”) performance to meet some public sector systems' needs and/or private sector systems' needs. Photonic Integrated Circuits (“PICS”) with circuit densities over 100 optical elements are emerging quickly to replace optical subsystems in optical communication systems that have traditionally been served by individual bulk fiber optic components technology. Much like the revolution in electronics integration, photonics integration will only continue to evolve and expand in complexity and capability. However, the performance of PICS for use in analog RF photonic systems is too poor for PICs to make any significant inroads into analog applications. The performance of analog fiber optic versions of these systems fabricated using PICs is even further behind that of versions based on discrete fiber-based optical components. This is rooted in the lower performance available from the individual integrated devices that make up an optical link.
FIG. 1 shows a block diagram of a prior art, externally-modulated fiber optic link 10, the building block of many analog RF systems that utilize fiber optics. The link 10 consists of a continuous wave (“CW”) laser source 20 (with a wavelength λ) that is intensity modulated by an external modulator (“Mod”) 30 with an input RF signal, transported over an optical fiber 40 to a photodetector, e.g., output photodiode (“PD”) 50, where the optical signal is converted back to an RF signal. The RF gain of the link 10 in FIG. 1 is determined by only two parameters: 1) the efficiency of the external modulator, Vπ, denoted by the voltage required to induce a π-phase shift of the light in the external modulator 30, and 2) the photocurrent at the output which is a function of the power in the CW laser 20, the optical losses, and the quantum efficiency of the output photodetector 50. The gain of this link 10 is plotted in FIG. 2 as a function of the total received photocurrent for several modulator efficiencies (Vπ), wherein the output photodiode 50 contains a 50 Ohm parallel resistor to achieve a good RF output impedance match. As can be seen in the plot, the trend to achieving gain is to both decrease the modulator Vπ and increase the detected photocurrent. In fiber optic versions of this link, link gains greater than unity (0 dB) are readily achieved at microwave frequencies.
The noise figure of the link in FIG. 1 is usually the most important metric that is considered in link design. Noise degrades signal fidelity and degrades the ability to process the signals further. The noise figure of the link in FIG. 1 is plotted versus photocurrent for several modulator efficiencies (Vπ) in FIGS. 3A-3C. The three plots in FIGS. 3A-3C from left to right include plots of the shot-noise limited performance (RIN=0, solid curves) and plots including laser RIN of −155, −165 and −175 dBc/Hz, respectively (dashed curves), wherein RIN is the relative intensity noise of the laser 20. For similar reasons as link gain, the noise figure generally decreases as the modulator Vπ decreases and as the detected photocurrent increases. However, when RIN is included, eventually the noise figure approaches a minimum, the value of which increases with higher levels of RIN.
The difference between the solid and dashed curves in FIGS. 3A-3C at a fixed modulator Vπ is the degradation in noise figure performance due to the added laser RIN. The degradation is reasonably independent of modulator Vπ, as RIN manifests itself as added noise in the output RF signal, independent of modulator efficiency until the link achieves a very low noise figure. Typical semiconductor lasers achieve RIN values in the −155 to −165 dBc/Hz range or worse, depending on frequency. Laser RIN is therefore an important factor to consider in achieving low noise figures.
One solution to mitigate the effects of laser RIN is to cancel it in the output of a balanced link. FIG. 4 shows an example of this type of prior art link 12. A dual-output (complementary) modulator 32 is utilized that provides outputs that are 180 degrees out of RF phase. Both signals are then propagated down two identical fibers 40 (“Delay/Remote”) where the outputs are subtracted in a balanced pair of photodiodes 52 (“Bal PD”). Because the laser RIN is common-mode and the RF signal of interest is complementary, the subtraction process suppresses the effects of RIN by an amount determined by the amplitude and phase match of the two paths. It is typical for this type of prior art link 12 to achieve 20 to 30 dB of common-mode rejection. However, this requires two identical length fibers. Obtaining identical length fibers is difficult to implement for long delays and at higher frequencies, wherein environmental drifts cause dynamic phase mismatches, especially due to thermal mismatches in the two propagation paths, thus limiting the level of common-mode rejection.
After the noise figure, the second important metric for an RF link is the spurious response characterized by the spur-free dynamic range. For any real system, as the input signal level increases, the output eventually compresses. This compression results in nonlinearity or spurious signals in the output. For narrow bandwidth systems, the third-order intermodulation distortion products are usually the most important, as they remain in-band with the original signals of interest and cannot be filtered out. A typical transfer diagram to show this is plotted in FIG. 5. The linear response of a system is extrapolated with a straight line (i.e., a linear response) having slope of 1. The third-order distortion is fit to a straight line having a slope of 3, as the power in these distortion products is proportional to the cube of the fundamental power. The intersection of these two straight lines is the intercept point. When referenced to the output power, this intercept point is the output third-order intercept (“OIP3”). If the noise floor is included as in FIG. 5, the third-order, spurious-free dynamic range (“SFDR”) is defined as the range of input powers for which the third-order distortion remains below the noise floor. The SFDR is uniquely determined by the noise floor (which in turn is determined by noise figure) and the OIP3 of the link.
The OIP3 of a Mach-Zehnder modulated link is plotted in FIG. 6 along with the 1-dB compression point and the saturated output power. In this plot, the distortion introduced by the 50 Ohm impedance-matched photodetector is assumed negligible. The OIP3 of this type of link is only a function of photocurrent; therefore, the link SFDR is increased by increasing photocurrent, so long as the noise floor remains low, which implies a low noise figure. In a similar fashion to noise figure, if the laser RIN is high, the SFDR saturates and no longer increases as photocurrent increases as the OIP3 and the noise floor increase photocurrent together, capping SFDR.
To increase the OIP3 beyond that predicted by FIG. 6, linearization techniques must be employed to either mitigate or improve the distortion characteristics of the modulator, or to limit the input signal level. Improvements using these approaches have been done in the prior art. One such approach is to limit the input signal level with an array of elements as shown in FIG. 7 for amplifiers. By splitting the input signal level into many lower power copies using RF splitters 60, 62 (two being shown in FIG. 7 for ease of understanding), the input signal level is reduced at the input to each identical amplifier 70. This lowers the operating point of the amplifier 70. This is shown graphically by the two operating points in FIG. 5. By lowering the input power from operating point 1 to operating point 2 (in this example, n=10 resulting in a 10 dB lower input power), the third-order distortion reduced by an amount that is three times smaller than the linear response (30 dB in this example) owing to the third-order distortion having a slope of 3. After amplification, the signals are then summed coherently (amplitude and phase-matched) in a second RF splitter (used as a combiner). Because both the signal and distortion products add linearly in the second RF splitter, the net result is that the distortion at the RF output is lower (20 dB in this example), and the gain of the entire system is equal to the gain of a single amplifier. Therefore, a configuration like that shown in FIG. 7 is trading off the use of n identical amplifiers to yield a system with the same gain as a single amplifier, but with an OIP3 that increases by an amount equal to 10*log(n)(10 dB in this example) because the OIP3 increase is equal to 50% of the net reduction in the distortion for systems with equivalent gain.
Arraying optical links can also be used to increase their performance metrics. Arrayed photodetectors increase link gain. A single modulator can be split into many photodetectors, and depending on the recombination method, can result in higher link gain. An arrayed optical link, for example, includes an RF splitter and multiple photodetectors hard-wired together. Another optical link, for example, includes a WDM. If a prior art optical link 14 is used as shown in FIG. 8, instead of an amplifier 70 of FIG. 7, the link linearity can be improved at the expense of more parallel channels. This is impractical, but technically feasible, with fiber-based components owing to the added cost and complexity of utilizing multiple links. The gain of the apparatus in FIG. 8 is degraded due to the initial RF splitter 60, when RF recombination is considered. If hard-wired photodetectors 54, 56 are used (e.g., output photodetector leads being connected together), the gain is improved by 10*log(n), albeit at the expense of lowering the bandwidth of the photodetectors due to the parallel nature of the photodetector capacitances summing to yield a lower RC-limited bandwidth. Arrayed photodetectors also have been used to improve link linearity, when photodetector distortion is the limiting factor.